1. Field of the Invention
The present invention relates to a crystallization method of an amorphous silicon thin film, and more particularly, to a method of forming a thin film transistor (TFT), which is applied to flat panel displays, such as liquid crystal displays (LCDs) and light emitting displays (LEDs), 3-dimensional very large-scale integrated (VSLI) semiconductor devices, etc., using the crystallization method, a TFT manufactured using the method, and a flat panel display including the TFT.
2. Description of the Related Art
Generally, an amorphous silicon thin film transistor (a-Si TFT) used in a flat panel display, such as a liquid crystal display (LCD), an organic or inorganic light emitting display (OLED or LED), etc., uses amorphous silicon for its semiconductor active layer including a source, a drain, and a channel, and provides a low electron mobility of 1 cm2/Vs or less. Accordingly, such an a-Si TFT has been recently replaced with a polycrystalline silicon TFT (poly-Si TFT). A poly-Si TFT provides greater electron mobility than the a-Si TFT and is stable to light irradiation. Thus, a poly-Si TFT can be used as a driving and/or switching transistor of active matrix LCDs or active matrix organic LEDs.
Typically, the poly-Si TFT is manufactured by directly depositing polycrystalline silicon, or by crystallizing amorphous silicon after a deposition thereof.
Examples of a method of directly depositing polycrystalline silicon include a chemical vapor deposition (CVD), a photo CVD, a hydrogen radical (HR) CVD, an electron cyclotron resonance (ECR) CVD, a plasma enhanced (PE) CVD, and a low pressure (LP) CVD.
Examples of a method of crystallizing amorphous silicon after a deposition include a solid phase crystallization (SPC), an excimer laser crystallization (ELC), a sequential lateral solidification (SLS), a metal induced crystallization (MIC), and a metal induced lateral crystallization (MILC).
However, the SPC is impractical to apply because it requires a long duration of a high temperature process at 600° C. or greater. While the ELC has an advantage of low-temperature crystallization, the intensity uniformity of a laser beam for crystallization, which is diverged by an optical system, is poor. On the other hand, the SLS involves irradiating a laser beam onto an amorphous silicon layer through a chevron pattern mask to form a localized region of crystallized polysilicon and requires a precise control of the laser irradiation. Moreover, it is difficult to reproduce such a polysilicon thin film having uniform characteristics with the SLS. The MIC has an advantage of low-temperature crystallization because a metal thin film acting as a catalyst for crystallization is deposited on a surface of an amorphous silicon layer prior to crystallization of the amorphous silicon layer. However, a polysilicon layer formed by the MIC contains small, disordered crystals, and the metal that remains in the polysilicon layer degrades the properties of a thin film transistor manufactured from it.
Recently, a MILC has been suggested as an alternative to resolve the problems arising with the above-described conventional amorphous silicon crystallization methods. The MILC induces lateral, sequential crystallization by reacting a metal with silicon to form silicide. In MILC, the metal used for crystallization of an amorphous silicon layer hardly remains in a semiconductor active layer, and the resulting crystals have a small size and are highly ordered. Therefore, a current leakage due to a remaining metallic component and other electrical degradations do not occur. Moreover, crystallization by the MILC can be induced at a relatively low temperature of 300-500° C.
FIGS. 1A to 1D illustrate a method of forming a polysilicon layer using such a conventional MILC method.
Referring to FIG. 1A, a buffer layer 2 is formed on a transparent substrate 1 using SiOx, and an amorphous silicon layer 3 is deposited on the buffer layer 2. An insulating layer 4 is deposited on the amorphous silicon layer 3, and a photosensitive layer 5, for example, a photoresist layer, having a predetermined pattern is formed. Next, as illustrated in FIG. 1B, the insulating layer 4 on the amorphous silicon layer 3 is etched into a predetermined pattern using the photosensitive layer 5, and a nickel (Ni) thin film 6 is deposited thereon as a catalyst for crystallization. Thereafter, a thermal process is carried out. As a result, as illustrate in FIG. 1C, an MIC region 7 is formed in a region A of the amorphous silicon layer 3, from which the insulating layer 4 is removed, as a result of a metal induced crystallization, and an MILC layer 8 is formed in a region B in which the insulating layer 4 remains unetched, as a result of a lateral crystallization from the MIC region 7. Next, the insulating layer 4 is removed from the MILC region 8 to provide a polysilicon thin film, as illustrated in FIG. 1D.
Where such a MILC is applied to form a TFT, after a formation of a gate dielectric layer and a gate electrode on an amorphous silicon thin film, a Ni thin film is formed thereon and thermally processed for crystallization. Alternatively, after a formation of a gate dielectric layer and a gate electrode, a photoresist is applied to cover the gate dielectric layer, the gate electrode, and partially cover a source region and a drain region, and a Ni thin film is formed thereon. Next, the photoresist is removed for crystallization through a thermal process. In the above cases, a boundary of resulting MIC and MILC regions, which have different crystalline structures, is not aligned with a source/channel boundary and a drain/channel boundary. Accordingly, characteristics of the channel region are negatively enhanced.
Furthermore, as described above, the conventional MILC requires lengthy operations including a photoresist layer formation for a larger MILC region and a crystallization catalyst deposition, thereby complicating the manufacture of a TFT.